In digital modulation schemes used in recent years in wireless communications, such as mobile telephony and LAN (Local Area Network), a modulation format such as QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) is employed.
In this type of modulation format, generally a signal trajectory undergoes amplitude modulation at a time of inter-symbol transition, and a high frequency modulation signal superimposed on a carrier signal of a microwave band has a signal amplitude (envelope) which changes with time. A ratio of a peak power and an average power of a high frequency modulation signal is referred to as PAPR Peak-to-Average Power Ratio).
In order to ensure high linearly, an amplifier which amplifies a signal with a large PAPR needs to be supplied with sufficiently large power from a power supply, so that a waveform of the signal is not distorted even at a peak power. In other words, it is necessary to operate the amplifier in a region of power sufficiently lower than saturation power limited by a power supply voltage, with power margin (back off).
In general, since a linear amplifier which operates in class A or B, has the maximum efficiency in a vicinity of a saturation power, an average efficiency is lowered when the linear amplifier operates in a region where the back off is large.
In an Orthogonal Frequency Division Multiplexing (OFDM) system using a multicarrier employed in next generation Mobile telephony, wireless LAN, and digital television broadcasting, PAPR is further increased, and an average efficiency of an amplifier is further lowered.
Therefore, it is desirable that a characteristic of an amplifier has a high efficiency even in a power region with a large back off.
As a system for amplifying a signal with high efficiency and with a wide dynamic range in a power region with a large back off, a transmission system referred to as Envelope Elimination and Restoration (EER) is proposed by L. Kan in Non-Patent Document 1 (Proceedings of the I.R.E, pages 803-806, 1952). In this system, first an input modulated signal is separated into a phase component and an amplitude component thereof. The phase component with a constant amplitude is supplied to the amplifier while maintaining phase modulation. At this time, a high frequency amplifier operates in the vicinity of saturation at which efficiency is always largest.
On the other hand, the amplitude component undergoes power amplification with high efficiency using a class D amplifier or the like, while maintaining amplitude modulation, and the amplitude component is supplied as intensity modulated power (modulated power supply) to the amplifier.
When operated in this way, the amplifier operates as a multiplier, the phase component and the amplitude component of the modulated signal are combined, and an output modulated signal amplified with high efficiency not depending on the back off, is obtained.
As a system that resembles the EER scheme, a method referred to as Envelope Tracking (ET) is also known. For example, in Non-Patent Document 2 (Microwave Symposium Digest, 2000 IEEE MITTS Digest, Vol. 2, FIG. 1, pp. 873-876), an example thereof is reported.
The ET scheme has in common with the EER scheme a configuration in which an amplitude component of an input modulated signal is power-amplified with high efficiency using such as a class D amplifier, while maintaining amplitude modulation and is supplied as modulated power to the amplifier.
A point of difference is that in the EER scheme only a phase modulated signal with amplitude constant is supplied to the amplifier and saturation operation performed, whereas in the ET scheme the input modulated signal including both the amplitude modulation and the phase modulation is as it is supplied to the amplifier which operates linearly.
In the ET scheme, since the amplifier operates linearly, efficiency is lower than in the EER scheme. However, since only the minimum necessary power is supplied, in accordance with an amplitude size, the ET scheme can obtain a high efficiency, as compared to a case of using the amplifier at a constant voltage not depending on an amplitude.
The ET scheme has an advantage in that a timing margin in which an amplitude component and a phase component are combined, is relaxed, and the implementation is easier in comparison with the EER scheme.
In the EER scheme and the ET scheme, an amplitude component is converted to a pulse modulation signal, and switching amplification is performed using a class D amplifier or the like.
Regarding a pulse modulation scheme, a pulse width modulation (PWM) scheme has been generally used. In Patent Document 1 and Patent Document 2, a configuration is proposed in which a delta modulation scheme (or a Pulse Density Modulation (PDM) scheme), which is superior in linearity, is applied.
In recent years, a sigma-delta modulation scheme or the like, in which Signal to Noise Ratio (SNR) is enhanced, has been used.
In wireless communication systems using digital modulation in recent years, such as mobile telephony and the like, standards have been set in which Adjacent Channel Leakage Power Ratio (ACPR) and Error Vector Magnitude (EVM) representing modulation error should be kept at or below a constant value.
In the EER scheme and the ET scheme, an operation frequency band of a class D amplifier or pulse modulator forming a modulation power supply must at least be twice or more as large as a band of a modulated signal, in order to satisfy the specifications.
For example, in mobile phone WCDMA (Wideband Code Division Multiple Access) specifications, the modulation band is approximately 5 MHz. In the IEEE 802.11a/g standards for wireless LAN, the modulation band is approximately 20 MHz.
In general, since high power and high speed switching amplification is difficult to implement, it is difficult to realize such a wide band modulation power supply.
A configuration shown in FIG. 15 is proposed in Patent Document 3 as a simplest method of modulating power supplied to the amplifier in accordance with an amplitude component of a modulated signal.
This method includes supplying steadily an average power (voltage) to an amplifier, and supplying excess power (voltage) to the amplifier only when an amplitude becomes greater than or equal to a fixed value.
Operation of this method will be described using FIG. 15 and FIG. 16. In this method, a voltage Bc is steadily applied to an amplifier (RF AMP) 204. The voltage Bc is lower than an output peak voltage because the average power is provided.
When an envelope sensor (EES) 201 detects a peak at which an envelope (amplitude component) 10 of an input modulated signal is higher than a reference voltage Vref, a power valve 203 is turned ON, and an excess voltage Bv is applied to the amplifier 204.
As a configuration of the power valve 203, a method using capacitive coupling is proposed in Patent Document 4, and a method having combined usage of capacitive coupling and magnetic coupling is proposed in Patent Document 5.
With this type of configuration, an unnecessary power is not supplied in a region where the amplitude component of the input modulated signal is small, and it is possible to increase average efficiency of the amplifier.